According to data just released by the National Center for Health Statistics, teen birth rates fell in 2011 to an historic low of 31.1 births per 1,000 young women ages 15 to 19. This represents a 25 percent drop since 2007 and a 49 percent drop since teen birth was at its highest in 1991 when there were 61.8 births per 1,000 young women aged 15 to 19.

The report, which will be featured in the March issue of the Pediatrics, does not look at the potential reasons for these declines though many other studies have examined teen sexual behavior and contraception use over the past couple of decades. Sexual behavior data for 2011 is not available but studies in recent years have shown that fewer teens are having sexual intercourse especially at young ages. Still, most experts point to improved contraceptive use (and improved contraceptive methods) as the key to this decline.

“We have gone from a social norm where you don’t use contraception at first sex to where you do. Lots of study shows that using contraception at first sex begins a pattern of using it down the road.”

She went on to point to a number of policies that may have impacted teens’ use of contraception including new guidelines that recommend IUDs and Implants (two of the methods with the lowest failure rates) for teens, health care reform changes that lower the cost of birth control, and the new CDC guidelines that suggest teens do not need pelvic exams before being prescribed the birth control pill or other hormonal methods of contraception. Of this last policy, Lindberg said this:

“We think that’s lowered what we call the psychic barrier to getting prescription contraception methods. For teenaged girls that first (exam) can be frightening.”

Both Lindberg and Brady Hamilton, a statistician who led the research for this report, noted that other factors such as the economy may also play a role. Hamilton explained that:

“Choices about whether to engage in risky behavior very much hedges on consideration of the consequences.”

“I think there has been a lot of hunkering down that may have changed the dynamic …that teens are giving to their decisions. It may be hard to get pregnant if you are living in your parents’ basement.”

The 20-year decline, however, may have also led to a change in cultural norms around teen childbirth. Hamilton and his team calculated that if the rates had stayed at their peak there would have been 3.6 million additional babies born to teens since 1991. By preventing these births in young women who are now in their twenties and thirties, we may have helped changed the thinking of today’s teens.

Still there is more work to be done. The teen birth rate in the United States remains the highest in the industrialized world. In 2010 (when the U.S. rate was 37.9 per 1,000 young women), the rate in Russia was 30.2, in Britain it was 24, and in Switzerland it was just 4.3 births per 1,000 teen women.

If enacted, new policies such as the American College of Obstetrician and Gynecologists’ (ACOG) recommendation for making birth control available over the counter and the American Academy of Pediatrics’ (AAP) recommendation that doctors provide prescriptions for Emergency Contraception on a “just in case” basis could go a long way toward lowering these rates even further.